This invention relates to the hydroformylation of a lower alkene to produce an aldehyde having one carbon atom more than the alkene feedstock by a process which comprises reacting a mixture of hydrogen and carbon monoxide with the alkene in the presence of a catalyst which comprises rhodium complexed with a triorganophosphine as exemplified by triphenylphosphine, which is particularly useful and with which the present invention is primarily concerned. Hydrogen and carbon monoxide also make up part of the complexed rhodium catalyst. The reaction is conducted in the presence of a liquid reaction medium comprising a high-boiling solvent.
The invention is specifically directed to those reaction systems, which are well known in the pertinent prior art, in which the hydrogen, carbon monoxide, and vapors of the alkene are sparged through the liquid reaction medium during the course of the reaction both to effect agitation of the reaction medium and also to strip out the aldehyde product as it is formed, in the vaporous mixture exiting the hydroformylation reactor.
The literature dealing with hydroformylation reaction systems of the type just described is voluminous. U.S. Pat. No. 3,527,809, to Pruett et al., provides a comprehensive discussion of the chemistry of these systems. U.S. Pat. No. 3,239,566, to Slaugh et al., is also pertinent as background for the basic process.
U.S. Pat. No. 4,151,209, to Paul et al., provides a discussion of product recovery by stripping of the liquid reaction medium and also discusses applicable high-boiling reaction solvents. U.S. Pat. No. 4,480,138, to Hackman et al., also discusses especially useful reaction solvents and deals at considerable length with the technology of the product-stripping operation itself.
U.S. Pat. No. 4,148,830, to Pruett et al., teaches the use of high-boiling reaction by-products as the reaction solvent.
The recovery of the aldehyde product from the liquid reaction medium can be effected in a number of ways including continuously drawing off a slip stream from the hydroformylation reactor and distilling it to separate relatively low-boiling compounds including the aldehyde product as an overhead stream and then returning the stripped residue to the hydroformylation reactor as desired. Alternatively, the continuous stripping of the reaction medium by sparging hydrogen, carbon monoxide, and alkene vapors into the liquid contained in the hydroformylation reactor, with the aldehyde product being continuously withdrawn from the top of the reactor in the exiting gases, is particularly useful when the aldehyde is of substantial volatility, as in the case of propionaldehyde and the butyraldehydes. The continuous hydroformylation of propylene in a gas-sparged reactor in this manner is discussed by Hershman et al. in "Industrial and Engineering Chemistry Product Research and Development," Vol. 8 (1969) pages 372-375.
Finally, U.S. Pat. No. 4,247,486, to Brewester et al., describes a system in which the gas recycle through the hydroformylation reactor is controlled in such a manner as to maintain the liquid level in the reactor and control the build-up of high molecular weight by-products.
The potential problem of loss of valuable rhodium in the reactor overhead in the form of entrainment is mentioned in U.S. Pat. No. 4,247,486, and also in U.S. Pat. No. 4,287,369, to Harris et al., who also describe a product-recovery system in which the gases exiting the hydroformylation reactor are subjected to condensation with non-condensed components being recycled to the hydroformylation reactor. Both U.S. Pat. No. 4,247,486 and U.S. Pat. No. 4,286,369 disclose the use of demisting pads through which the gases exiting the hydroformylation reactor are passed in order to remove entrained liquid droplets for return to the reactor. In neither case is there a suggestion that the demisting pads are in any way deficient in preventing loss of rhodium from the reaction system.
It has now been discovered. however, that conventional entrainment separators such as the demisting pads disclosed in the prior art just described are not completely effective in preventing loss of rhodium into the reactor overhead system. Even small losses of rhodium through the entrainment-separation system are very significant economically because of the high cost of rhodium. Also, of course, rhodium is a strategically-important metal regardless of its monetary cost. It is not known with certainty whether these losses of rhodium through the, for example, demisting pads occur as very fine entrainment which is not trapped by the pads or whether there is actually some volatilization of the rhodium as complexes which have an appreciable, although certainly very low, volatility.
It is, accordingly, an object of the present invention to provide a method for recovering that portion of the rhodium which exits the top of the hydroformylation reactor and which is not trapped by conventional demisting equipment. It is also another object to recover this fugitive rhodium substantially free from high-boiling reaction by-products which, if returned to the hydroformylation reactor, tend to act as catalyst deactivators. Other objects will be apparent from the following detailed description.